One of the methods to increase a wireless communication system throughput is the usage of two orthogonal polarizations than can be done without significant sophistication of the transceiver. This enables a simultaneous transmission and/or receipt of two data flows, each flow being carried by orthogonally polarized electromagnetic waves. The key components of such a system are dual polarized antennas and orthomode transducers, which are devices used to distinguish appropriately polarized waves from the common dual-pol antenna port. Each of single-pol waves is guided to one or more orthomode transducers ports.
Traditionally, orthomode transducers are provided on hollow metallic waveguides, and more specifically on several waveguides of rectangular cross-section which support only one polarization, and one waveguide of a round or square cross-section, which supports two types of orthogonal waves. Usually, a round or square waveguide port is connected to the dual polarized antenna. One of such orthomode transducers is disclosed in U.S. Pat. No. 6,087,908 and presented herein in FIG. 1. The main drawbacks of such orthomode transducer are large dimensions, complexity and, consequently, high manufacturing cost since the high-precision milling or welding of several complex metal pieces is required. It leads to the fact that it is hard to integrate such waveguide orthomode transducer into the microwave front-end of the transceiver devices.
To overcome the abovementioned drawbacks of the orthomode transducer based on hollow metallic waveguides, planar orthomode transducers based on probes were recently introduced. In this case, separation of orthogonal polarizations is provided by orthogonal metal probes etched on a single or several printed circuit boards which are fixed across a round or square dual polarized waveguide. This waveguide is connected to the dual polarized antenna port. Similar orthomode transducer is disclosed in patent JPH11308004 and shown in FIG. 2. In such structure both of orthomode transducer output ports are provided as planar (usually 2 microstrip) transmission lines on the printed circuit board. Wherein probes are just extensions of these microstrip lines inside a waveguide.
Technology of printed circuit boards is developed rapidly in the last decade. The printed circuit boards process for low-loss substrates which have low dielectric losses in microwave frequency range has became much cheaper than manufacturing of hollow waveguide components and, thus, more profitable in mass-production. From the technical point of view, the usage of printed circuit boards instead of waveguides allows significantly decreasing device dimensions and simplifying its integration in the transceiver's radio frequency circuit.
However, the probe based orthomode transducer implemented on the printed circuit board has several other drawbacks. These are a relatively low isolation between orthogonally polarized channels, a narrow frequency band and the need of additional quarter-wave metal backshort for the input of dual polarized waveguide.
Alternatively, an orthomode transducer can be embodied by replacement of the dual polarized waveguide (with round or square cross-section) connecting antenna output and the orthomode transducer itself with the antenna element that can be either a separate dual polarized antenna or a primary radiator element that forms the required amplitude and phase distributions to illuminate the main antenna (e.g., a lens antenna or a dish reflector antenna).
Such approach is disclosed in patent CN104752841 and shown herein in FIG. 3. In this case, a primary radiator is a resonant cavity loop antenna (330) which is implemented in the top metallization layer of the printed circuit board. Besides this, due to the loop antenna of square form, it is possible to simultaneously excite the loop with both linear orthogonal polarizations. Substrate integrated waveguide sections (335, 336) are used as feeding lines for the loop antenna (330). The substrate integrated waveguide is a planar transmission line that is provided inside the printed circuit board in between two metallized layers by bounding the waveguide channel area with metallized via holes. To achieve a high isolation between two transducer's outputs, a via hole (334) is arranged in the center of the primary radiator. This via hole prevents resonant loop antenna excitation from the isolated orthogonal substrate integrated waveguide. In this design, the primary radiator forms the required amplitude and phase distributions for the overlying main antenna. The substrate integrated waveguides of orthogonal polarizations (335, 336) can be ended by transitions to microstrip lines to facilitate integration with a radio frequency transceiver. Wherein both the primary radiator and the transceiver can be implemented on the same printed circuit board.
But there are several technical drawbacks in the orthomode transducer described above and considered as the closest prior art of the present invention:
1. Lack of versatility. The usage of the primary antenna element together with the main antenna significantly decreases the variety of possible antenna designs which can be implemented together with the primary antenna element. In other words, the considered orthomode transducer does not have a commonly used dual polarized port such as, for example, a dual polarized metal waveguide. This prevents from using different antennas with the orthomode transducer and significantly limits its applications. Such approach cannot be implemented for very common horn antennas or integrated lens antennas which have waveguide feed radiator placed on the flat lens surface;
2. Narrow frequency band. As such, a resonant cavity slot antenna is a narrow conductive frame which starts to radiate at the tuning frequency that depends on the frame perimeter. That is why both the antenna and, consequently, the orthomode transducer have relatively narrow operational frequency range (4%) that has been confirmed by experimental data as presented in CN104752841;
3. Low isolation between orthogonal polarizations. An experimental orthogonal polarization isolation of the described orthomode transducer is only 22 . . . 23 dB in the operational frequency band (data also shown in CN104752841). For many wireless communication applications this isolation could be not enough;
4. High insertion loss. The considered orthomode transducer with the primary antenna element has relatively high insertion loss due to the narrow radiating slot of the resonant cavity loop frame. A narrow slot in the loop frame increases the surface current density and the electric field intensity in dielectric near to the slot. This increases the insertion loss level.
Thus, there is a need for a more universal orthomode transducer device which is applicable for connection to any dual polarized antennas with waveguide interface, that has wider passband, higher orthogonal polarization isolation, lower insertion losses and is simple and planar.